1. Field of the Invention
The present invention relates to a digital signal receiving tuner and particularly to a digital signal receiving tuner receiving digital television broadcast signals.
2. Description of the Background Art
With respect to cable television (CATV), introduction of HFC (Hybrid Fiber Coax) is now proceeding that includes coaxial cables as CATV drop wires to respective homes and a trunk network formed of optical fibers. HFC is introduced with the purpose of providing a broadband data communication service to each home with the rate of several M bits/sec. With 64 QAM (Quadrature Amplitude Modulation), a high-speed data line can be produced having a bandwidth of 6 MHz and a transmission rate of 30 Mbits/sec where a cable modem is used. Any empty channel of the cable television system can be utilized to achieve high-speed data communication at the rate from 4 Mbits/sec to 27 Mbits/sec.
A digital set-top box (STB) is a tuner for CATV. The cable modem allows a downstream data signal transmitted from a CATV station to be displayed on a television monitor, while the STB allows a downstream data signal from the CATV station that has been modulated by QPSK (Quadrature Phase Shift Keying) to branch from the tuner so that the signal is processed by a CPU to be output to a personal computer. The cable modem thus transmits a downstream data signal over an empty channel of CATV with a band extending from 54 MHz to 860 MHz while the STB uses frequencies in another band extending from 70 MHz to 130 MHz.
Current ground wave analogue television broadcasts are delivered by using both of the VHF and UHF bands. Three types of digital television broadcastings have globally been started in 1998-2000. In Japan, a full-scale ground wave digital broadcasting will be started through the UHF band in 2003. For almost all input streams for digital broadcasting to the STB, MPEG-2 is used. For outputs of the digital STB, television receivers are used regardless of ground wave broadcasting, satellite broadcasting and cable television broadcasting. Thus, the digital STB generally has the same structure.
On the other hand, according to types of services and service providers, different front-end circuits, CA (Conditional Access) systems, software types depending on the system of data broadcasting service, digital interfaces for connection with external equipment and the like are used.
FIG. 4 is a schematic block diagram of a conventional cable modem. Referring to FIG. 4, a received signal is supplied to a high-pass filter (HPF) 1 serving as an IF filter where low-frequency components are eliminated and then the signal is supplied to input switching circuits 2, 14 and 22 to be switched to HIGH BAND, MID BAND and LOW BAND. HPF 1 has an attenuation range from 5 to 42 MHz and a pass band of 54 MHz or higher. HIGH BAND, MID BAND and LOW BAND correspond respectively to bands of 470-860 MHz, 170-470 MHz and 54-170 MHz. However, respective ranges are not particularly limited to the specific examples above.
In general, input switching circuits 2, 14 and 22 are switching circuits formed of switching diodes or band division filters for switching. Each band enters an operating state according to a received channel so that other bands do not operate. Specifically, when a channel of HIGH BAND is received, the UHF band is selected so that HPF 1, input switching circuit 2, a radio frequency (RF) amplification input tuning circuit 3, an RF amplifier 4, an RF amplification output tuning circuit 5, a mixer circuit 6, a local oscillator circuit 7, an IF amplifier circuit 19, a SAW (surface acoustic wave) filter 20, an IF amplifier circuit 21 and a PLL (phase-locked loop) channel select circuit 27 operate, and remaining MID BAND (VHF HIGH BAND) circuits namely input switching circuit 14—mixer circuit 18 and a local oscillator circuit 13 as well as LOW BAND (VHF LOW BAND) circuits namely input switching circuit 22—mixer circuit 26 and a local oscillator circuit 8 stop operating.
Similarly, when a MID BAND channel is received, HPF 1, input switching circuit 14, RF amplification input tuning circuit 15, RF amplifier 16, RF amplification output tuning circuit 17, mixer circuit 18, local oscillator circuit 13, IF amplifier circuit 19, SAW filter 20, IF amplifier circuit 21 and PLL channel select circuit 27 operate while input switching circuit 2—mixer circuit 6, local oscillator circuit 7, input switching circuit 22—mixer circuit 26 and local oscillator circuit 8 stop operating.
When a LOW BAND channel is received, HPF 1, input switching circuit 22, RF amplification input tuning circuit 23, RF amplifier 24, RF amplification output tuning circuit 25, mixer circuit 26, local oscillator circuit 8, IF amplifier circuit 19, SAW filter 20, IF amplifier circuit 21 and PLL channel select circuit 27 operate while input switching circuit 2—mixer circuit 6, local oscillator circuit 7, input switching circuit 14—mixer circuit 18, and local oscillator circuit 13 stop operating.
Operations are carried out successively by transmitting channel select data from a CPU (not shown) to PLL channel select circuit 27, a channel is then selected and simultaneously, the band switching circuit operates in accordance with band information. Power supply to circuitry for each band is switched by the band switching circuit and each function is controlled accordingly.
The cable modem operates as described below. A received signal is passed through HPF 1 via an input terminal 11. Input switching circuits 2, 14 and 22 switch the signal to any band circuitry and RF amplification input tuning circuits 3, 15 and 23 select a channel. Then, the RF signal of its corresponding channel is amplified by RF amplifiers 4, 16 and 24, and the RF signal is tuned by RF amplification output tuning circuits 5, 17 and 25 and thus the received signal is output.
Mixer circuits 6, 18 and 26 and local oscillator circuits 7, 13 and 8 convert the frequency of the corresponding RF signal output from RF amplification output tuning circuits 5, 17 and 25 into the intermediate frequency to generate an intermediate-frequency signal to be supplied to IF amplifier circuit 19. The intermediate-frequency signal is passed through SAW filter 20 and amplified again by IF amplifier circuit 21 to be output from an output terminal 12.
For the conventional cable modem shown in FIG. 4, tracking adjustments are necessary in RF amplification input tuning circuits 3, 15 and 23, RF amplification output tuning circuits 5, 17 and 25 and local oscillator circuits 7, 13 and 8. The adjustments are required for each of HIGH BAND, MID BAND and LOW BAND.
Specifically, the tracking adjustments should be made by using air-core coils for resonance inductors of RF amplification input tuning circuits 3, 15 and 23, RF amplification output tuning circuits 5, 17 and 25 and local oscillator circuits 7, 13 and 8 each, so that the frequency is variable. The adjustments are necessary for each of HIGH BAND, MID BAND and LOW BAND, resulting in a longer adjustment time and a higher operation cost.
The air-core coils included in local oscillator circuits 7, 13 and 8 are each fixed to a substrate for example by adhesive or the like. In order to confirm the fixed state of the coils, the cable modem is lightly struck by a rubber hammer. A problem then arises that shock noise is generated due to frequency change caused by vibration of the struck air-core coils and noise is further generated by piezoelectric effect generated from a chip-type ceramic capacitor because of the striking.